Fluid-filled sac (4)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fluid-filled sac</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cyst that I've seen before include "Benign growth" , "Bag-like growth containing diseased matter" , "Abnormal body cavity" , "Bladder" , "Membrane" .)